1.\" Copyright (c) 1983, 1991, 1993 2.\" The Regents of the University of California. All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 3. All advertising materials mentioning features or use of this software 13.\" must display the following acknowledgement: 14.\" This product includes software developed by the University of 15.\" California, Berkeley and its contributors. 16.\" 4. Neither the name of the University nor the names of its contributors 17.\" may be used to endorse or promote products derived from this software 18.\" without specific prior written permission. 19.\" 20.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 21.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 22.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 23.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 24.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 25.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 26.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 27.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 28.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 29.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 30.\" SUCH DAMAGE. 31.\" 32.\" @(#)flock.2 8.2 (Berkeley) 12/11/93 33.\" $FreeBSD$ 34.\" 35.Dd December 11, 1993 36.Dt FLOCK 2 37.Os 38.Sh NAME 39.Nm flock 40.Nd "apply or remove an advisory lock on an open file" 41.Sh LIBRARY 42.Lb libc 43.Sh SYNOPSIS 44.In sys/file.h 45.Fd "#define LOCK_SH 0x01 /* shared file lock */" 46.Fd "#define LOCK_EX 0x02 /* exclusive file lock */" 47.Fd "#define LOCK_NB 0x04 /* don't block when locking */" 48.Fd "#define LOCK_UN 0x08 /* unlock file */" 49.Ft int 50.Fn flock "int fd" "int operation" 51.Sh DESCRIPTION 52The 53.Fn flock 54system call applies or removes an 55.Em advisory 56lock on the file associated with the file descriptor 57.Fa fd . 58A lock is applied by specifying an 59.Fa operation 60argument that is one of 61.Dv LOCK_SH 62or 63.Dv LOCK_EX 64with the optional addition of 65.Dv LOCK_NB . 66To unlock 67an existing lock 68.Dv operation 69should be 70.Dv LOCK_UN . 71.Pp 72Advisory locks allow cooperating processes to perform 73consistent operations on files, but do not guarantee 74consistency (i.e., processes may still access files 75without using advisory locks possibly resulting in 76inconsistencies). 77.Pp 78The locking mechanism allows two types of locks: 79.Em shared 80locks and 81.Em exclusive 82locks. 83At any time multiple shared locks may be applied to a file, 84but at no time are multiple exclusive, or both shared and exclusive, 85locks allowed simultaneously on a file. 86.Pp 87A shared lock may be 88.Em upgraded 89to an exclusive lock, and vice versa, simply by specifying 90the appropriate lock type; this results in the previous 91lock being released and the new lock applied (possibly 92after other processes have gained and released the lock). 93.Pp 94Requesting a lock on an object that is already locked 95normally causes the caller to be blocked until the lock may be 96acquired. 97If 98.Dv LOCK_NB 99is included in 100.Fa operation , 101then this will not happen; instead the call will fail and 102the error 103.Er EWOULDBLOCK 104will be returned. 105.Sh NOTES 106Locks are on files, not file descriptors. 107That is, file descriptors 108duplicated through 109.Xr dup 2 110or 111.Xr fork 2 112do not result in multiple instances of a lock, but rather multiple 113references to a single lock. 114If a process holding a lock on a file 115forks and the child explicitly unlocks the file, the parent will 116lose its lock. 117.Pp 118The 119.Fn flock , 120.Xr fcntl 2 , 121and 122.Xr lockf 3 123locks are compatible. 124Processes using different locking interfaces can cooperate 125over the same file safely. 126However, only one of such interfaces should be used within 127the same process. 128If a file is locked by a process through 129.Fn flock , 130any record within the file will be seen as locked 131from the viewpoint of another process using 132.Xr fcntl 2 133or 134.Xr lockf 3 , 135and vice versa. 136.Pp 137Processes blocked awaiting a lock may be awakened by signals. 138.Sh RETURN VALUES 139.Rv -std flock 140.Sh ERRORS 141The 142.Fn flock 143system call fails if: 144.Bl -tag -width Er 145.It Bq Er EWOULDBLOCK 146The file is locked and the 147.Dv LOCK_NB 148option was specified. 149.It Bq Er EBADF 150The argument 151.Fa fd 152is an invalid descriptor. 153.It Bq Er EINVAL 154The argument 155.Fa fd 156refers to an object other than a file. 157.It Bq Er EOPNOTSUPP 158The argument 159.Fa fd 160refers to an object that does not support file locking. 161.El 162.Sh SEE ALSO 163.Xr close 2 , 164.Xr dup 2 , 165.Xr execve 2 , 166.Xr fcntl 2 , 167.Xr fork 2 , 168.Xr open 2 , 169.Xr lockf 3 170.Sh HISTORY 171The 172.Fn flock 173system call appeared in 174.Bx 4.2 . 175